Cosmology at the turning point of relativity revolution. The debates during the 1920's on the "de Sitter Effect".

This thesis is devoted to a historical reconstruction of the cosmological debates which took place during the 1920's on the so-called de Sitter effect. Such a theoretical redshift-distance relation played a fundamental role in the early phases of modern cosmology, when the concept of the expanding universe still had to enter the scientific study of the universe as a whole. The de Sitter effect represents the linking thread between the predictions of theoretical relativistic models of Einstein and de Sitter and observational cosmology. The history of those debates permits therefore to understand how cosmology developed passing from a sphere of theoretical speculations to a truly empirical science.